[sdiy] TH XR2206 VCO tune pot

[Derek Holzer]

Thanks everyone so far who's made a stab at explaining this one to me 
(Oscar, Scott, Dave, David)! It would have been one thing to throw a pot 
in there and come up with some measurement, but another to understand 
*why* that measurement is what it is.

David G. Dixon wrote:
> Thomas Henry's designs always allow the full range of voltage on the coarse
> tuning pot.  It's a philosophical choice.  I think he assumes the builder
> will customize the range to suit his own needs.  For example, some may wish
> to use the VCO as an LFO, while others may only want to use the VCO within
> the audio range.

I'm using a pair of switched caps for this VCO, so that it's audio VCO 
in one position and VC-LFO in the other.

Oscar asked if I knew the 0V frequency of the oscillator--in fact I 
don't because I haven't started the tuning process yet. I wanted to get 
the range under control first.

It's unlikely that I will be using some sort of keyboard or 
MIDI-to-Frequency controller with these oscillators any time soon, but 
in the event I did I assume that any 0V frequency can be compensated for 
with the right amount of offset voltage.

So why would having control over the 0V frequency be important 
otherwise? So that your pot starts in the middle of the audible range....?

> If you are using a +/-15V PSU, then with no resistors you will get the full
> 30V range out of the pot.  At 1V/octave, that's a range of 30 octaves!  If
> you want to reduce the range to, say, 10 octaves, and assuming you're using
> a 100k pot with a 100k input resistor to the summing node, then you must
> either
> 
> a) put 200k of additional resistance between the rails and the pot, thus
> making the total rail-to-rail resistance equal to 300k, or 
> 
> b) put a 300k resistor between the pot and the summing node.  

This matches the math of what Oscar wrote as well, as long as I'm clear 
about one thing:

In option b), the 100K summing resistor is *replaced* by the 300K 
resistor, correct?

Clear as Guinness! ;-)
D.

-- 
::: derek holzer ::: http://blog.myspace.com/macumbista ::: 
http://www.vimeo.com/macumbista :::
---Oblique Strategy # 198:
"Instead of changing the thing,
change the world around it."